Conventional underwater probes are suspended in a body of water, carried onboard a vessel, or are allowed to rest upon the floor of a body of water. The measurements that these probes provide can be used to determine the nature of noise emission from underwater sources such as those generated from natural (weather, geological, or biologically-related) or man-made processes. For instance, the probes can be used to detect distant vessels, perform seismic surveys, monitor ambient noise in the sea, aid in fish localization, or aid in fossil fuel exploration. Such probes can also be used to assess the effectiveness and directionality of radiation from underwater sources.
An acoustic intensity measurement provides an indication of the vector energy flux density of an acoustic field. Acoustic intensity is the product of acoustic particle velocity, {right arrow over (u)}, and acoustic pressure, p. There are two primary probe design types known in the art for measuring underwater acoustic intensity: pressure-gradient probes and inertial probes.
The most commonly used design is the pressure-gradient probe. Such a probe is described in U.S. Pat. No. 4,982,375. This probe consists of two hydrophones fixed a distance Δr apart. One key advantage of this design type is that the hydrophones can be rigidly fixed in space (motion is not central to sensing). The sum of the outputs provides a direct measurement of the acoustic pressure at the midpoint of the hydrophones. Although the acoustic velocity is not directly measured using this technique, it can be determined using a finite-difference approximation to the velocity, given as:
      u    r    ≈      j    ⁢                  Δ        ⁢                                  ⁢        p                    ω        ⁢                                  ⁢        ρΔ        ⁢                                  ⁢        r            where ur is the acoustic velocity; Δp is the difference between the hydrophone outputs; ρ is the density of water; and ω is the radian frequency. The acoustic intensity is then determined as the time-averaged product of the acoustic pressure and velocity.
The pressure-gradient probe has several inherent disadvantages. The derived velocity is a function of frequency, and therefore, each spectral component to be studied must be weighted differently to compensate for the frequency dependence. The distance the hydrophones are set apart can cause inaccurate measurements. Setting the hydrophones too closely together greatly decreases the dynamic range of the probe. Setting the hydrophones too far apart can result in the inaccurate measurement of acoustic fields which depart from perfect plane wave behavior.
A second design type which is rapidly gaining popularity in the art is the inertial probe design. The output of an inertial probe is directly related to the motion of the probe body. Two such probes are described in U.S. Pat. Nos. 5,392,258 and 6,172,940. The probe of U.S. Pat. No. 5,392,258 measures the acoustic particle velocity directly from the output of a geophone encased in a neutrally buoyant syntactic foam cylindrical body. The probe is suspended such that motion in directions other than the primary sensing direction of the geophone is suppressed. Pressure hydrophones, attached to the end caps of the cylinder, are spatially averaged to yield acoustic pressure. The product of the averaged hydrophone output and the velocity sensor output is directly proportional to the acoustic intensity. U.S. Pat. No. 6,172,940 combines the inertial probe design above with the gradient technique. Two geophones are encased in independent, neutrally buoyant bodies and set a fixed distance, Δr, apart. The average of the two geophone signals gives a signal proportional to the particle velocity at the midpoint of the line separating the two geophones. In this case, the acoustic pressure is not directly measured, but again it can then be determined using the measured geophone output signals.
Other inertial-type probes which claim to directly measure underwater acoustic intensity are disclosed in U.S. Pat. Nos. 3,311,873 and 2,582,994. The probe of U.S. Pat. No. 3,311,873 directly measures both acoustic pressure and acoustic acceleration electrically integrated to determine velocity. The probe of U.S. Pat. No. 2,582,994 is an air-tight, rigid, metal sphere encapsulating an acoustic velocity sensor. While both of these probes may potentially measure acoustic intensity, the probe of U.S. Pat. No. 3,311,873 disregards the inertial effects of water which seriously impedes the motion of the sensor to an acoustic force and the probe of U.S. Pat. No. 2,582,994 is extremely large and perturbs the acoustic field to a significant degree, causing scattering of wave motion.
In any embodiment of an inertial probe design, suspension design is critical, since the signal output is proportional to the motion of the probe body. Fundamentally, the suspension must fix the time-averaged position of the probe while permitting movement of the sensor body with the acoustic force. It should have a natural frequency well below the intended range of operation. Finally, it must not distort the probe response in magnitude or phase. For single-dimension probes, as in U.S. Pat. Nos. 5,392,258 and 6,172,940, suspension designs are straightforward. Two-dimensional probes are much more common in underwater acoustics. In this case, probe motion can be restricted to be planar with the third axis used as a support axis. The navy class Directional Frequency Analysis and Recording (DIFAR) sonobouy does this by supporting the probe vertically in the water column from a taut cable connected to a surface float package, allowing unconstrained motion in the sensing plane (a plane normal to the suspension cable and parallel to the water surface).
Designs sensitive in three-dimensions are difficult to build without significantly affecting the probe's response. U.S. Pat. No. 6,370,084 describes a three-axis inertial type probe encased in a viscoelastic rubber which serves as the suspension. However, special care must be exercised in the design of the shear stiffness of such a suspension. If the design relies on shear stiffness to control the suspension resonance, the mount must be designed to avoid any axis being constrained by compression of the material since the compressional modulus is normally much higher. With such a system, it is extremely difficult to avoid considerable anisotropy in the suspension.